Recent progress in the synthesis ofregio-isomerically pure bis-adducts of emptyand endohedral fullerenes†

Maira R. Ceróna and Luis Echegoyenb*

Bis-functionalized fullerenes have resulted in very effective acceptors in organic photovoltaic solar cells. Here we present ashort overview of specific and recently reported pure bis-adduct regioisomers of C60 using short tethers, a few recentexamples of bis-adducts of C70 and finally examples of bis-adducts of endohedral cluster fullerenes. We also discuss theunexpected reactivity differences of empty fullerenes (C60 and C70) compared with those of trimetallic nitride clusterendohedral fullerenes (M3N@Ih-C80, M= Sc, Lu) towards tethered-bis-addition reactions. Finally, we describe the remarkableendohedral-cluster regiochemical control of multiple additions. Copyright © 2016 John Wiley & Sons, Ltd.

Keywords: empty fullerenes; endohedral fullerenes; bis-functionalization of fullerenes; tethered method

INTRODUCTION

Since the serendipitous discovery of the fullerenes in 1985,[1] andthe first detection of endohedral fullerenes almost immediatelyafter,[2] the first functionalization reactions were reported[3–5] assoon as the fullerenes became available in macroscopic amountsin 1990.[6] These derivatization reactions lead to products withsubstantially improved solubility and modified chemical andphysical properties, increasing their potential applications inmany different fields.[7–10] Almost invariably, functionalizationleads to the formation of multiple adducts, as it is very difficult,if not impossible, to control the conditions in order to limit thereactivity to a single addition. As will be discussed in thesucceeding texts, both mono-functionalization and bis-functionalization can lead to compounds which act as excellentelectron acceptors in organic photovoltaic (OPV) devices, yield-ing relatively high photoconversion efficiencies (PCEs). Giventhat multiple isomeric bis-adducts can be formed with C60 andC70, developing reagents and techniques to limit the accessiblepossibilities has led to a renaissance in this field, which was orig-inally pioneered many years ago by Diederich et al.[11,12] In thisoverview article we describe recent developments in theregiochemical control of bis-functionalizations of empty (C60and C70) and of some endohedral cluster fullerenes.We begin by briefly describing the number of statistically pos-

sible bis-adducts of C60 and C70 and describe a nomenclaturethat we recently introduced to systematically name the C70 com-pounds.[13–15]

Possible bis-adduct regioisomers

Empty fullerenes possess two different types of bonds, the [5,6]-bonds between a five-membered and a six-membered ring and[6,6]-bonds between two six-membered rings (Fig. 1a).[1] Themost reactive bonds on an empty fullerene are typically the[6,6]-junctions,[16] thus multiple adducts of fullerenes normallyinvolve [6,6]-bonds.[11,15,17] If the two addends are identical and

the additions occur exclusively on [6,6]-bonds, the statisticalnumber of possible regioisomers is 8 in the case of C60 (refer toFig. 1b for the nomenclature),[15] and 64 in the case of C70 (referto Fig. 1c for the nomenclature).[13]

Regioisomers observed

It has been shown that bis-adduct formation on C60 is not statis-tical, and in most cases the trans-3 and e-isomers are the pre-ferred products. If there is very low steric congestion betweenthe addends, the cis-1 isomer is the preferred product. Afterthe formation of the mono-adduct, no matter what the natureof the addend is, the cis-1 and e-bonds are the shortest;therefore, addition to these bonds is preferred. In addition, bis-adducts cis-1, e and trans-3 possess enhanced HOMO-LUMOcoefficients, which explain the preferential formation of thesespecific regioisomers. For the case of C70, the typically preferredisomers involve adducts on α-bonds on opposite poles of the D5h

symmetric compound because the α-bonds are the shortest andmost strained. Thus, the 3 isomers that are normally obtained arethe α-6-α′, the α-7-α′, and the α-9-α′ following our recentlyintroduced nomenclature (previously known as the 12-o’clock,2-o’clock and 5-o’clock isomers, respectively)[13,18] The

* Correspondence to: Luis Echegoyen, University of Texas at El Paso Department ofChemistry, CCSB #2.0304, 500 W. University Avenue El Paso, TX, USA 79968-0509.E-mail: echegoyen@utep.edu

† This article is published in Journal of Physical Organic Chemistry as a specialissue on New Latin American Conference at Villa Carlos Paz, Córdoba,Argentina, 2015 by Elba I. Buján (Universidad Nacional de Córdoba)

a M. R. CerónChemistry, University of Texas at El Paso, CCS-Bldg Room 3.0709 500 W. Uni-versity Avenue, El Paso, TX, USA, 79968-0519

b L. EchegoyenCollege of Science, University of Texas at El Paso, Bell Hall Room 100 500 W.University Avenue, El Paso, TX, USA, 79968-0509

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Special issue article

Received: 23 November 2015, Revised: 28 February 2016, Accepted: 4 March 2016, Published online in Wiley Online Library: 2 May 2016

(wileyonlinelibrary.com) DOI: 10.1002/poc.3563

J. Phys. Org. Chem. 2016, 29 613–619 Copyright © 2016 John Wiley & Sons, Ltd.

nomenclature uses Greek letters to define the addition positionsand a number that corresponds to the smallest number of bondsseparating the additions sites. Same type bonds on oppositesides across the δ-bonds are differentiated using a primedesignation.[13]

Methods to control bis-adduct regioisomer formation

The challenge in the synthesis of bis-adducts is to obtainregioisomerically pure products that avoid complicated, costly,and inefficient purification methods such as HPLC.[14,19–21] Thereare very few methods that provide control for the formation ofspecific bis-adduct regioisomers. The first to be reported andthe one that continues to be the most general and effectivewas described by Diederich et al. in 1994, named tether-directedmultifunctionalization.[11] This method uses a variable lengthand rigidity tether that connects two or more reactive centersto direct the location of the additions on the fullerene surface,and it is still widely used and will be described in more detaillater in this article, using new reagents.

The second method was reported by Krӓutler et al. in 1996 andconsists of a rather unique and very interesting topologicallycontrolled reaction which lacks general applicability and has onlybeen shown to work with C60 and more recently with C70(Fig. 2).[22,23] The method uses molten anthracene under vacuumat 240 °C in the presence of the corresponding fullerene andyields exclusively the trans-1 bis-Diels–Alder adduct in 48% yieldwith C60 (Fig. 2a) and exclusively the α-6-α′ bis-adduct in 68%yield with C70 (Fig. 2b). This topologically controlled bis-additionreaction is completely specific, and no other isomers are obtainedfor either C60 or C70 and the products can be used to further

functionalize these fullerenes regioselectively, using what Prof.Kräutler has described as orthogonal transposition,[24,25] whereother groups are added to the bis-adducts, followed by thermalretro Diels–Alder of the anthracenes to afford compounds thatare otherwise very difficult, if not impossible, to prepare in largequantities without observing multiple isomeric forms.[26–31]

Why are fullerene bis-adducts important?

As briefly mentioned earlier, recently prepared bis-functionalizedfullerenes have resulted in very effective acceptors in OPV solarcells.[32–38] PCEs up to 10.6% have been achieved using isomericmixtures of fullerene bis-adducts, such as indene-C61-bis-adducts(IC61BA) in a solution processed tandem solar cell.[33] Althoughnot all isomerically pure bis-regioisomers have been tested in so-lar cell devices, most of the reported cases have demonstratedthat pure regioisomers many times perform better than the cor-responding isomeric mixtures,[39–43] although some exceptionshave also been noted.[21,41,44] These observations justify the ne-cessity to synthesize pure bis-adducts, and this in turn has ledto a renaissance of the methods and reagents available to de-crease the number of fullerene multiadduct derivative isomers.Recent activity in this area has resulted in new and

regioisomerically pure bis-adducts that have performed reason-ably well in OPV devices.[37,40,42] In some cases, even when usingthe tether-directed method, the number of isomers obtained isstill rather large, for example, in the case of the synthesis of teth-ered bis-PC61BM (phenyl butyric acid methyl ester-C61)

[45] andtethered bis-DPM-C60 (diphenyl methano)[44] for which the num-ber of isomers obtained was 7 and 12, respectively. The mainreasons for these observations are the lack of symmetry of theaddend, in which case diastereomers can also be observed and

Figure 1. (a) Two different types of bonds on empty fullerenes. First addition site is designated with a black bond b) nomenclature for the eight pos-sible regioisomers of C60; (c) some examples of bis-adduct regioisomers of C70 relevant to this work

Figure 2. Topologically controlled bis-anthracene derivatives of (a) C60(trans-1 isomer)[22]; (b) C70 (12 o’clock or α-6-α′ isomer)[23]

Figure 3. Pure bis-adducts synthesized using the tether-directedmethod (a) IC61BA cis-2 isomer[40]; (b) IC61BA α-2-α isomer[42]; (c) N-methyl-phenyl-C61-propyl-2-fulleropyrrolidine cis-1a isomer[37]

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the extreme flexibility of the tether used.[37] It is known that theuse of shorter tethers and rigid addends decreases the numberof observed isomers. To the best of our knowledge, the only purebis-adducts synthesized using the tether-directed method andapplied to OPV solar cells are the recently reported IC61BA cis-2isomer,[40] IC71BA α-2-α isomer,[42] and the N-methyl-phenyl-C61-propyl-2-fulleropyrrolidine cis-1a isomer[37] with 1% and 2%higher PCEs than the corresponding isomeric mixtures usingP3HT as the donor (refer to Fig. 3 for the structures).

Recent results

This article is not meant to be a comprehensive review of fuller-ene bis-adducts in general, but rather an account of specific andrecently reported pure bis-adduct regioisomers of C60 usingshort tethers, a few recent examples of bis-adducts of C70 and fi-nally examples of bis-adducts of endohedral cluster fullerenes.Part of the reason for introducing short tethers was to preparemore polar fullerene derivatives for enhanced solubility and easeof OPV device fabrication and also to explore C70 derivativeswhere both addends are attached to one pole of the D5h

symmetric compound, because most bis-adducts reported forC70 are located on opposite ends of the rugby-shaped molecule,on α- and α′-bonds.The tether-directed method has been implemented using

different types of reactions, such as addition-elimination (Bingel),1,3-dipolar, Diels–Alder and diazo cycloadditions.[46,47] Here welimit the examples for the most part to 1,3-dipolar and Bingelcycloadditions, with some exceptions, specifically employingthe following reagents shown in Fig. 4.

Tethered bis-1,3-dipolar cycloadditions to C60 and C70

We recently reported the regioselective tethered bis-1,3-dipolarcycloaddition of a bis-ylide reagent to C60 (Fig. 4a), by using ashort and rigid bis-aldehyde (o-phthalaldehyde). This decreased

the number of possible regioisomers from 8 to 2, but becauseof the presence of two chiral centers on the bis-ylide the statisti-cal number of possible diastereomers is 6 (Fig. 5). Interestingly,we only observed the formation of three regioisomericbis-adducts out of the six possible isomers. The structures ofthe products were unambiguously assigned by X-ray crystallog-raphy and by spectroscopic techniques as two meso forms ofthe cis-2 isomer (compounds 1 and 2 in Fig. 6) and the racemicmixture (RR and SS) of the cis-1 isomer (compound 3 in Fig. 6).[48]

We also reported the unexpected isomerism of the twomeso-forms upon thermal treatment of compound 1 at 180 °Cin o-dichlorobenzene solutions and the resulting quantitativeconversion to compound 2. We proposed a retro-1,3-dipolar cy-cloaddition reaction mechanism to account for the interconver-sion process, followed by migration of the corresponding ylideto a different cis-2 bond, which yields compound 2 exclusivelywith an inverted stereochemistry (Fig. 6).[48]

The redox properties of compounds 1, 2, and 3 werepreviously reported but not discussed.[48] Cyclic voltammetry

Figure 4. Reagents used to perform 1,3-dipolar, Bingel and diazo cycloadditions (a) tethered bis-ylide; (b) long tether bis-malonate; (c) short tether bis-malonate; (d) tethered bis-diazo

Figure 5. Structure of the six possible diastereomers that can be obtained from the 1,3-dipolar cycloaddition of a bis-ylide (Fig. 4a) to C60

Figure 6. Reported bis-C60-pyrrolidine regioisomers[48]

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and square wave voltammetry were measured indichloromethane solutions using 0.1M n-Bu4NPF6 as supportingelectrolyte. The cyclic voltammetry of compounds 1, 2, and 3showed three reduction waves with electrochemically reversiblebehavior. The first, second, and third reduction potentials ofderivatives 1, 2, and 3 were cathodically shifted compared withthe values for C60 (Table 1).[48] It is well known that addition toa double bond on C60 catholically shifts the reduction byapproximately 100mV.[49] Because 1, 2, and 3 are bis-adducts,cathodic shifts around 200mV were anticipated and compounds1 and 3 exhibit shifts of 240 and 247mV, respectively.Compound 2 exhibited a shift of 305mV (Table 1).[48] Thus,higher open circuit voltages (Voc) could be expected for thesecompounds in OPV solar cells.

We have applied the same tethered bis-1,3-dipolarcycloaddition reaction on C70 and obtained four regioisomericbis-adducts, which are currently being purified andcharacterized. It is clear from the results observed with C60 andC70 that the 1,3-dipolar cycloaddition reaction is not particularlyregioselective because it leads to the formation of threeregioisomers in the case of C60 and four in the case of C70, anobservation that has been previously noted.[8,50–52]

Tethered bis-Bingel cycloadditions to C60 and C70

Because the short-tethered 1,3-dipolar bis-cycloadditions werenot particularly regioselective, we also explored reactionsinvolving different bis-malonate tethered addends (Fig. 4b andc) with C60 and C70, in which case not only regioisomers butdiastereomers can be observed, depending on the orientationof the ethoxycarbonyl groups (in–in, in–out, and out–out).[53,54]

Using a long and rigid tethered moiety with C60 we observedonly one product out of the 24 possible isomers that wasassigned by spectroscopic techniques, symmetry considerationsand DFT calculations as the trans-2 regioisomer with theethoxycarbonyl groups (EtO2C–) in an out–out orientation(compound 4; Fig. 7).[55] For the case of the less symmetricD5h-C70 we obtained two products, a Cs-symmetric bis-adductassigned as the α-6-α′ regioisomer and a C1-symmetricbis-adduct assigned as the α-7-α′ regioisomer based onspectroscopic data and DFT calculations, both with the EtO2C–groups in an out–out orientation (compounds 5 and 6; Fig. 7,respectively).[55]

Interestingly, when we used a previously reported short tethermoiety to perform a bis-Bingel cycloaddition on C60 and C70 (Fig.4c), we observed a pronounced difference in the reactivityexhibited by the fullerenes.[12,53,55] For C60 the cis-2 isomer wasthe only one observed (compound 7; Fig. 7),[12,53] but using C70instead of C60 did not lead to the formation of bis-adducts butto a dumbbell compound as the major product in 54% yield(compound 8; Fig. 7).[55] All attempts to obtain C70 bis-adductswith this reagent were unsuccessful. These results indicate thatafter the first addition to a C70 molecule, which preferentiallyoccurs at an α-bond, the length and rigidity of the tether woulddirect a second addition at a γ-bond, but these bonds are highlyunreactive. Therefore, the α mono-adduct reacts preferentiallywith another α-bond on a different C70 molecule, leading pre-dominantly to the dimeric dumbbell compound.[55]

Figure 7. Bis-Bingel regioisomers of C60 and C70[55]

Table 1. Redox potentials (V)a of 1, 2, 3 and C60

Compound C60 Isomer 1 Isomer 2 Isomer 3

E0/� �0.977 �1.218 �1.282 �1.224E �/�2 �1.369 �1.574 �1.658 �1.608E�2/�3 �1.817 �2.158 �2.197 �2.222E�3/�4 �2.253 — — —

aValues obtained by square wave voltammetry in V versus Fc/Fc+.

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Tethered bis-diazo cycloadditions to C60 and C70

Because of the higher thermal stability and redox reversibility ofmethanofullerenes compared with those of the pyrrolidine orBingel derivatives, we explored the functionalization of C60 andC70 using a bis-diazo reagent. Diazo cycloaddition reactions cangenerate two types of derivatives: the kineticallycontrolled product defined as a fulleroid, which involves additionat a [5,6]-bond, and the thermodynamic product defined as amethanofullerene, which results from addition ata [6,6]-bond.[56–58] Fulleroid derivatives usually isomerize quicklyto the more stable methanofullerene derivatives via a π-methanetransposition.[59,60] We recently designed a short, rigid, andsymmetric bis-diazo addend and reported the regio-selectivesynthesis of easily isolable and highly sterically congestedbis-derivatives of C60 and C70 with both additions on a commonhexagonal face.[13]

We observed that when the addition occurred at two[6,6]-bonds, a Cs-symmetric bis-adduct was obtained for C60

(compound 9, cis-1 isomer; Fig. 8) and Cs- and C1-symmetricbis-adducts for C70 (compounds 10 and 11, α-1-α and α-1-βisomers, respectively; Fig. 8). Somewhat unexpectedly, whenthe addition occurred at two [5,6]-bonds on one hexagon ringon C60, forming a bis-fulleroid, we observed that theCs-symmetric derivative underwent fast photooxygenation inair, to yield a 12-membered ring opened bis-fulleroid (com-pounds 12 and 13, respectively; Fig. 8).[13]

Endohedral cluster fullerene bis-adducts

Given that there is only one example of an OPV solar cell basedon an endohedral fullerene mono-adduct acceptor (PCBH-Lu3N@Ih-C80) and that reasonable efficiencies were obtained,[61]

there seem to be many possibilities for many rooms in the explo-ration of new endohedral fullerene derivatives as potential ac-ceptors in solar cells. Thus, we were interested in exploring the

Figure 8. Previously reported bis-methano derivatives and bis-fulleroids of C60 and C70[13]

Figure 9. Attempts to use tether-controlled bis-addition reactions with endohedral cluster fullerenes M3N@Ih-C80 (M = Sc and Lu)[62]

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synthesis and regiochemistry of bis-additions to endohedralfullerenes.

Because the tethered bis-1,3-dipolar, bis-Bingel, and bis-diazocycloadditions exhibited reasonable degrees of regioselectivityand gave rise to easily isolable bis-adducts of [60]- and [70]-ful-lerenes,[3,13,48,55] we decided to use the same reactions toprepare bis-adducts of Sc3N@Ih-C80 and Lu3N@Ih-C80. Interest-ingly and totally unexpectedly, we discovered that the reactionsthat worked so well with C60 and C70 did not proceed at all withthe trimetallic nitride endohedral fullerenes; we were only ableto observe the formation of mono-adducts in all cases (Fig. 9).The obvious explanation for these observations is that theencapsulated cluster must exert a very strong directing effect, to-tally preventing the addition of the second tethered group. Thisapparent directing control coming from the cluster inside led usto consider independent (non-tethered) bis-additions, an experi-ment that should have naturally preceded the tethered one.[62]

Independent (non-tethered) bis-additions to endohedral ful-lerenes have been reported before but very few details abouttheir regiochemistry have been discussed.[63–68] We recentlydescribed the statistically possible number of bis-adduct isomersthat could in principle be formed on an Ih-C80 cage, assumingthat the two addends are identical and that both [5,6] and [6,6]additions can occur. This analysis predicts that the total numberof possible isomers is 91, of which 63 are C1-symmetric, 18 areCs-symmetric and 10 are C2-symmetric.[62]

Yamakoshi et al. recently reported regioselective bis-1,3-dipolar cycloaddition reactions using M3N@Ih-C80 (M=Y andGd) and described one predominant and unsymmetricalbis-pyrrolidine isomer, which corresponds to one of the 63possible C1-symmetric bis-adducts.[69] The particular structurethat was proposed was based on computational studies, butgiven that there are 63 possible isomeric forms, the specificdefinitive assignment is somewhat tentative, because thecalculated energy differences are very small between differentisomers. This group also reported that using M3N@Ih-C80(M= Sc and Lu) did not result in the formation of substantialamounts of bis-adducts to be isolated and characterized.[69]

Fortunately, using the same bis-1,3-dipolar cycloaddition reac-tion with M3N@Ih-C80 (M= Sc and Lu) in our hands resulted in theformation and successful isolation of five new bis-adducts, threefor Sc3N@C80 and two for Lu3N@C80 (Fig. 10).[62] We were verylucky to observe that out of these five bis-adduct isomers, fourhad either Cs or C2 symmetry, thus lowering the number ofpossible structures and providing a more definitivecomputationally-aided assignment of the specific structures,mainly based on 1H-NMR spectroscopy. For Sc3N@C80 we

obtained one C1-symmetric, one Cs-symmetric, and one C2-sym-metric bis-adduct. For Lu3N@C80 we obtained one Cs-symmetricand one C2-symmetric bis-adduct (Fig. 10). Remarkably, for bothSc and Lu endohedral fullerenes, we observed the same C2-sym-metric bis-adduct and these are the first examples of intrinsicallychiral endohedral compounds, because of the resultingsymmetry based solely on the addition pattern, not on thepresence of chiral centers on the addends.[62]

Based exclusively on the NMR observations, we assigned thestructure of the Cs-symmetric isomer for Lu3N@C80 to one of onlyfour possible [5,6]-[6,6] bis-adducts, with mutually perpendicularpyrrolidines. To the best of our knowledge this is the first mixed(hybrid) bis-addition compound ever observed for anendohedral system, a rather unanticipated and very interestingresult.[62]

CONCLUSIONS

We described several reagents that were recently reportedwhich were designed for regioselective tethered bis-additionsto C60 and C70. These are part of a renaissance of the use ofthe tethered-controlled method to decrease the number ofbis-adduct isomers obtained for potential applications in OPVsolar cells. Different types of reactions, such as tetheredbis-Bingel, bis-1,3-dipolar, and bis-diazo cycloadditions, weredescribed, which lead to unique and highly sterically congestedbis-adducts of C60 and C70. The shorter tethers were designed toresult in more polar fullerene bis-adducts for enhanced solubilityand processability in the construction of OPV devices.We also described the interesting observation that the same

tethered-bis-addition reactions that work well with C60 and C70do not work at all with trimetallic nitride cluster fullerenes,indicative of very strong directing and controlling effects bythe encapsulated clusters on the sites of exohedral additions.Four of the five isolated and characterized bis-adducts ofSc3N@Ih-C80 and Lu3N@Ih-C80 using non-tethered bis-1,3-dipolarcycloaddition reactions unexpectedly and remarkably exhibitedrelatively high symmetry, two exhibiting an intrinsically chiraladdition pattern, and one exhibiting the first hybrid [5,6]-[6,6]bis-addition on a cluster fullerene. These results clearly show thatthe regiochemistry of multiple additions to endohedralfullerenes is strongly determined/controlled by the encapsulatedcluster and not by exohedral tethered multi additions.Given that the encapsulated clusters inside endohedral

fullerenes control the regiochemistry of the exohedralfunctionalizations, the next step in the field of

Figure 10. Addition patterns of the bis-adducts obtained for M3N@Ih-C80 (M = Sc and Lu)[62]

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bis-functionalization of endohedral fullerenes will involve thedesign of mixed metallic clusters to synthesize well-definedbis-adduct regioisomers.

Acknowledgements

L.E. thanks the NSF for generous support of this work undergrant (CHE-1408865) and to the NSF-PREM program (DMR-1205302). The Robert A. Welch Foundation is also gratefullyacknowledged for an endowed chair to L.E. (grant AH-0033).

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